l = ['5555_22_1_Иванов Иван Иванович.doc',
 '7777_33_1_Петров Петр Петрович.doc',
 '8888_21_2_Сидоров Степан Степанович.doc']
for i in l:
    lst = i.split('.')
    oldname = lst[0].split('_')
    newname = oldname[-1] + '_'+ '_'.join(oldname[:-1])+'.'+lst[1]
    print newname

 n = '5555_22_1_Иванов Иван Иванович.doc'
number = (n[:n.rfind('_')])
name = n[n.rfind('_')+1:n.rfind('.')]
expansion = n[n.find('.'):]
nev_n = '{}_{}{}'.format(name,number,expansion)
print(nev_n)

 n = '5555 22 1 Иванов Иван Иванович.doc'
expansion = n[n.find('.'):]
number, name = [], []
for i in n[:n.find('.')].split():
      number.append(i) if i.isdigit() else name.append(i)
print(' '.join(name+number)+expansion)